A demand for lower fuel consumption of vehicles is increasing in recent years for the purposes such as a reduction in the amount of CO2 emissions and protection of oil resources. For example, one approach to lower the fuel consumption is to reduce the viscous resistance of an engine oil by lowering its viscosity. However, lower viscosity causes problems such as oil leakage and seizure. In cold regions, startability at low temperatures is required. With regard to these issues, the US Society for Automotive Engineering (SAE) specifies the viscosity in the standard for Engine Oil Viscosity Classification (SAE J300). For 0W-20 grade oil, the high temperature high shear (HTHS) viscosity at 150° C. (ASTM D4683 or D5481) is specified to be Min. 2.6. In addition, for the same grade oil, the low temperature viscosity at −40° C. is specified to be 60,000 mPa·s or less with no yield stress (ASTM D4684) in order to ensure startability in cold regions. To lower the fuel consumption, there is a demand for an engine oil that satisfies the above standard and has a lower HTHS viscosity in the effective temperature at 80° C. or 100° C., and various viscosity index improvers have been suggested. Known examples of such viscosity index improvers include a methacrylic acid ester copolymer (Patent Literatures 1 to 4), an olefin copolymer (Patent Literature 5), and a comb copolymer (Patent Literatures 6 to 8).
However, these viscosity index improvers are insufficient in reducing the HTHS viscosity at 80° C. when added to an engine oil composition. Such an engine oil composition is susceptible to the viscosity reduction due to shear and exhibits a viscosity increase at low temperatures.